Parallel Execution -- 4 Parsing PX Queries

Unlike "regular" Serial Execution queries that undergo only 1 hard parse and multiple soft parses on repeated execution, Parallel Execution queries actually are ~~hard~~ parsed by each PX Server plus the co-ordinator at each execution. [Correction, as noted by Yasin in his comment : Not hard parsed, but separately parsed by each PX Server]

UPDATE 26-Apr-15: See Oracle Support Note 751588.1 and Bug 6274465 to understand how the PX Servers (PQ Slaves) also parse the SQL statement.

Here's a quick demo.

First, I start with a Serial Execution query.

[oracle@localhost ~]$ sqlplus hemant/hemant

SQL*Plus: Release 11.2.0.2.0 Production on Fri Apr 24 22:53:55 2015

Copyright (c) 1982, 2010, Oracle.  All rights reserved.


Connected to:
Oracle Database 11g Enterprise Edition Release 11.2.0.2.0 - Production
With the Partitioning, OLAP, Data Mining and Real Application Testing options

HEMANT>set serveroutput off
HEMANT>alter table large_table noparallel;

Table altered.

HEMANT>select count(*) from large_table;

  COUNT(*)
----------
   4802944

HEMANT>select * from table(dbms_xplan.display_cursor);

PLAN_TABLE_OUTPUT
------------------------------------------------------------------------------------------------------------------------------------
SQL_ID  5ys3vrapmbx6w, child number 0
-------------------------------------
select count(*) from large_table

Plan hash value: 3874713751

--------------------------------------------------------------------------
| Id  | Operation          | Name        | Rows  | Cost (%CPU)| Time     |
--------------------------------------------------------------------------
|   0 | SELECT STATEMENT   |             |       | 18894 (100)|          |
|   1 |  SORT AGGREGATE    |             |     1 |            |          |
|   2 |   TABLE ACCESS FULL| LARGE_TABLE |  4802K| 18894   (1)| 00:03:47 |
--------------------------------------------------------------------------


14 rows selected.

HEMANT>select
  2  executions, parse_calls, invalidations, sql_fulltext
  3  from v$sqlstats
  4  where sql_id = '5ys3vrapmbx6w';

EXECUTIONS PARSE_CALLS INVALIDATIONS SQL_FULLTEXT
---------- ----------- ------------- --------------------------------------------------------------------------------
         1           1             0 select count(*) from large_table

HEMANT>
HEMANT>select count(*) from large_table;

  COUNT(*)
----------
   4802944

HEMANT>select
  2  executions, parse_calls, invalidations, sql_fulltext
  3  from v$sqlstats
  4  where sql_id = '5ys3vrapmbx6w';

EXECUTIONS PARSE_CALLS INVALIDATIONS SQL_FULLTEXT
---------- ----------- ------------- --------------------------------------------------------------------------------
         2           2             0 select count(*) from large_table

HEMANT>
HEMANT>select count(*) from large_table;

  COUNT(*)
----------
   4802944

HEMANT>select
  2  executions, parse_calls, invalidations, sql_fulltext
  3  from v$sqlstats
  4  where sql_id = '5ys3vrapmbx6w';

EXECUTIONS PARSE_CALLS INVALIDATIONS SQL_FULLTEXT
---------- ----------- ------------- --------------------------------------------------------------------------------
         3           3             0 select count(*) from large_table

HEMANT>
HEMANT>select count(*) from large_table;

  COUNT(*)
----------
   4802944

HEMANT>select
  2  executions, parse_calls, invalidations, sql_fulltext
  3  from v$sqlstats
  4  where sql_id = '5ys3vrapmbx6w';

EXECUTIONS PARSE_CALLS INVALIDATIONS SQL_FULLTEXT
---------- ----------- ------------- --------------------------------------------------------------------------------
         4           4             0 select count(*) from large_table

HEMANT>
HEMANT>select count(*) from large_table;

  COUNT(*)
----------
   4802944

HEMANT>select
  2  executions, parse_calls, invalidations, sql_fulltext
  3  from v$sqlstats
  4  where sql_id = '5ys3vrapmbx6w';

EXECUTIONS PARSE_CALLS INVALIDATIONS SQL_FULLTEXT
---------- ----------- ------------- --------------------------------------------------------------------------------
         5           5             0 select count(*) from large_table

HEMANT>

5 executions with no additional parse overheads.

Next, I run Parallel Execution.

[oracle@localhost ~]$ sqlplus hemant/hemant

SQL*Plus: Release 11.2.0.2.0 Production on Fri Apr 24 23:04:45 2015

Copyright (c) 1982, 2010, Oracle.  All rights reserved.


Connected to:
Oracle Database 11g Enterprise Edition Release 11.2.0.2.0 - Production
With the Partitioning, OLAP, Data Mining and Real Application Testing options

HEMANT>set serveroutput off
HEMANT>alter table large_table parallel 4;

Table altered.

HEMANT>select /*+ PARALLEL */ count(*) from large_table;

  COUNT(*)
----------
   4802944

HEMANT>select * from table(dbms_xplan.display_cursor);

PLAN_TABLE_OUTPUT
------------------------------------------------------------------------------------------------------------------------------------
SQL_ID  4wd97vn0ytfmc, child number 0
-------------------------------------
select /*+ PARALLEL */ count(*) from large_table

Plan hash value: 2085386270

-----------------------------------------------------------------------------------------------------------
| Id  | Operation              | Name        | Rows  | Cost (%CPU)| Time     |    TQ  |IN-OUT| PQ Distrib |
-----------------------------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT       |             |       |  1311 (100)|          |        |      |            |
|   1 |  SORT AGGREGATE        |             |     1 |            |          |        |      |            |
|   2 |   PX COORDINATOR       |             |       |            |          |        |      |            |
|   3 |    PX SEND QC (RANDOM) | :TQ10000    |     1 |            |          |  Q1,00 | P->S | QC (RAND)  |
|   4 |     SORT AGGREGATE     |             |     1 |            |          |  Q1,00 | PCWP |            |
|   5 |      PX BLOCK ITERATOR |             |  4802K|  1311   (1)| 00:00:16 |  Q1,00 | PCWC |            |
|*  6 |       TABLE ACCESS FULL| LARGE_TABLE |  4802K|  1311   (1)| 00:00:16 |  Q1,00 | PCWP |            |
-----------------------------------------------------------------------------------------------------------

Predicate Information (identified by operation id):
---------------------------------------------------

   6 - access(:Z>=:Z AND :Z<=:Z)

Note
-----
   - automatic DOP: skipped because of IO calibrate statistics are missing


27 rows selected.

HEMANT>select
  2  executions, parse_calls, invalidations, sql_fulltext
  3  from v$sqlstats
  4  where sql_id = '4wd97vn0ytfmc';

EXECUTIONS PARSE_CALLS INVALIDATIONS SQL_FULLTEXT
---------- ----------- ------------- --------------------------------------------------------------------------------
         1           5             0 select /*+ PARALLEL */ count(*) from large_table

HEMANT>
HEMANT>select /*+ PARALLEL */ count(*) from large_table;

  COUNT(*)
----------
   4802944

HEMANT>select
  2  executions, parse_calls, invalidations, sql_fulltext
  3  from v$sqlstats
  4  where sql_id = '4wd97vn0ytfmc';

EXECUTIONS PARSE_CALLS INVALIDATIONS SQL_FULLTEXT
---------- ----------- ------------- --------------------------------------------------------------------------------
         2          10             0 select /*+ PARALLEL */ count(*) from large_table

HEMANT>
HEMANT>select /*+ PARALLEL */ count(*) from large_table;

  COUNT(*)
----------
   4802944

HEMANT>select
  2  executions, parse_calls, invalidations, sql_fulltext
  3  from v$sqlstats
  4  where sql_id = '4wd97vn0ytfmc';

EXECUTIONS PARSE_CALLS INVALIDATIONS SQL_FULLTEXT
---------- ----------- ------------- --------------------------------------------------------------------------------
         3          15             0 select /*+ PARALLEL */ count(*) from large_table

HEMANT>
HEMANT>select /*+ PARALLEL */ count(*) from large_table;

  COUNT(*)
----------
   4802944

HEMANT>select
  2  executions, parse_calls, invalidations, sql_fulltext
  3  from v$sqlstats
  4  where sql_id = '4wd97vn0ytfmc';

EXECUTIONS PARSE_CALLS INVALIDATIONS SQL_FULLTEXT
---------- ----------- ------------- --------------------------------------------------------------------------------
         4          20             0 select /*+ PARALLEL */ count(*) from large_table

HEMANT>
HEMANT>select /*+ PARALLEL */ count(*) from large_table;

  COUNT(*)
----------
   4802944

HEMANT>select
  2  executions, parse_calls, invalidations, sql_fulltext
  3  from v$sqlstats
  4  where sql_id = '4wd97vn0ytfmc';

EXECUTIONS PARSE_CALLS INVALIDATIONS SQL_FULLTEXT
---------- ----------- ------------- --------------------------------------------------------------------------------
         5          25             0 select /*+ PARALLEL */ count(*) from large_table

HEMANT>

Each of the 5 executions had parse overheads for each PX server.
Note : The 5 "PARSE_CALLS" per execution is a result of 4 PX servers. You might see a different number in your tests.

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